(1) Field of the Invention
This invention relates to a method for separating volatilizable contaminants, such as chlorinated and non-chlorinated solvents and the lighter fractions of petroleum hydrocarbons, from soil, and for recovering the contaminants for recycling or environmentally sound ultimate disposal.
(2) Description of the Related Art
Historically, soils contaminated by solvents, oils and similar organic materials through leakage from storage or piping, accidental spills or inadequate disposal have been managed in several ways. Representative of the approaches which have been taken are: (1) excavation and reburial in an approved landfill; (2) soil flushing using recovery and recharge wells; and (3) in-situ biological treatment using supplemental nutrients and possibly supplemental bacteria.
These methods have not been entirely satisfactory for a host of technical, legal and cost reasons. Excavation and reburial of even moderate size tracts of contaminated soil is enormously costly. Moreover, even properly designed and operated state-of-the-art landfills have been found to leak. Soil flushing has serious technical limitations in that very large volumes of water must be flushed through the soil to sufficiently lower contaminant concentrations. In-situ biological treatment also possesses drawbacks, among them its unsuitability in areas with low groundwater and its high cost.
An alternative method of treatment for soils containing easily volatilized contaminants was attempted by the U.S. Environmental Protection Agency ("EPA") in 1984, as reported in an article by D. Hazaga, S. Fields and G. P. Clemons, entitled "Thermal Treatment of Solvent Contaminated Soils," Alternative Technology, 1984 pp. 404-06. The contaminated soil, containing 1,1,1 trichloroethane, trichloroethene, toluene, ethyl benzene and ortho-, meta-, and para-xylene in concentrations from 10,000 to 110,000 ppb., was fed through an asphalt drying unit, also known in the industry as an aggregate dryer or rotary kiln. The drying unit included a liquid propane burner at one end which supplied combustion gases at 375.degree. F. to the interior of the dryer. From exposure to the 375.degree. F. gas, the contaminants were vaporized and the combined mixture of gases passed through a cyclone and bag house for removing particulate emissions, and then discharged directly to the atmosphere. The treated soil was discharged from the dryer, collected and then returned to the site.
Although the foregoing method was reported to be effective in reducing the concentrations of some contaminants by at least 99%, serious drawbacks were evident. Volatile organic compounds ("VOC's") removed from the soil being treated were discharged to the atmosphere, thus decontaminating one medium, the soil, at the expense of polluting another medium, the air. In order to dilute the concentration of emitted VOC's in the ambient air, and because of local air discharge requirements, the dryer could only be operated at a feed rate of 10-15 tons of soil per hour, rather than the design rate of 100 tons/hour. Localities with more stringent air quality regulations would necessitate an even lower feed rate. Furthermore, the dryer could only be operated in dry weather conditions to prevent the emitted VOC's from being scrubbed from the atmosphere by rainfall, and thus returned to the soil.
In addition, in the EPA method, the interior of the dryer was not isolated from combustion of the burner fuel, and therefore the possiblity of ignition and partial combustion of flammable volatilized contaminants existed. In fact, combustion of the volatilized contaminants was assumed to have occurred in the dryer. Partial combustion of many common contaminants produces toxic intermediate species. Thus, the problem of pollutant discharge to the atmosphere is compounded in the EPA method by the possible discharge of toxic gases.